Yes that could be one of the reasons FDA gave CRL.In terms of...

Yes that could be one of the reasons FDA gave CRL.
In terms of efficacy; it may be that Mesoblast presented data from the first 2 trials showing inadequate efficacy in the treated group. FDA however may grant approval to Mesoblast based on the subset that demonstrated efficacy i.e pediatric severe AVGHD.
CRL is a mechanism for FDA to limit approval to the pediatric subgroup until further gold standard data can be presented demonstrating efficacy.

"The data shows most efficacy-based deficiencies arose when the FDA identified a
population that failed to respond adequately to the drug candidate. Some applicants overcome
the deficiency by submitting results from additional efficacy studies that show increased or
sufficient efficacy in the challenged populations. Failure to overcome sufficient FDA-identified
deficiencies occur when an applicant fails to demonstrate efficacy in these populations or
produced additional data that confirmed the lack of efficacy in the challenged subpopulations. In
these cases, the FDA usually withholds approval of drug use in those subpopulations, but grants
approval for drug use in populations where efficacy is sufficiently demonstrated. It is important
to note that the instances of approval on Phase II Clinical data appear to be on the rise. However,
an approval on only Phase II data for efficacy as a response in a CRL must be compelling, and it
is likely that the safety profile of the product will have to be equally as compelling to receive
FDA approval."